1. Field of the Invention
The present invention relates to sensors for use in paper feeding machines such as copier, printers, and the like, and particularly to a sensor for indicating a paper supply condition in said machine.
2. Description of Related Art
Paper feeding machines which utilize cassettes such as removable paper trays to supply paper to the machine are plentiful in the art. Copiers, printers, facsimile machines, and other such devices have become prevalent in today's work place and their operation has become integral to the success of many businesses throughout the world. A typical machine takes stock paper from a tray and processes the paper in some way to yield the finished product at some output port. The supply of stock paper must be replenished from time to time as the paper in the cassette is used up. Essential to the operation of such machines is a uninterrupted supply of stock paper available on demand. Since the paper supply is typically located within the machine, and thus hidden from view, it becomes necessary to alert an operator when the paper supply is almost depleted so that the cassette can be refilled before the paper is exhausted. When the paper is supplied by a removable tray, the need for an indication of a low paper status is magnified because the storage capacity of the tray is typically less than the storage capacity of a dedicated compartment, and hence more likely to run out. For Example, Hewlett Packard manufactures several printers with removable trays having 250 sheet capacity and 500 sheet capacity, whereas some larger machines have dedicated compartments with storage for several thousands of sheets of paper.
In order to ensure a constant supply of paper, sensors have been developed to alert the machine's operator when the paper supply is low. Several attempts have been made to produce a low paper sensor which is readily adaptable to existing paper feeding machines, is small, lightweight, easy to manufacture, and reliable, but thus far no satisfactory sensor meets all of these goals.
Von Luhmann et al., U.S. Pat. No. 3,902,713, teaches a photoelectric device adapted for maintaining the height of a stack of papers. The reference shows a light source directed at a spherical or cylindrical object resting on the surface of the stack of papers and having a reflective surface. On the other side of the light source is a photosensitive device which receives the light reflected off the object when the stack of papers is at the correct height. As the paper is depleted, the angle of incidence from the object becomes such that light is no longer reflected to the photosensitive device, which signals a motor to increase the height of the stack of paper. The device is adapted primarily for small changes in the height of the stack of papers, although the object's path can be altered as paper is removed to increase or decrease the sensitivity of the device.
Lillibridge, U.S. Pat. No. 4,585,221 discloses a sensor for determining a low envelope condition in an envelope forming machine. The reference shows a light emitting and light receptor aligned and directed at a stack of envelopes, where the light reflected off the stack of envelopes from the light source is received by the light receptor as long as envelopes are in the chute. By placing the sensor at various places along the chute, the number of envelopes in the chute which will cause a low envelope condition to occur, i.e., no light reflected of the stack of envelopes, can be varies to suit the user. This sensor is not easily adaptable to present paper trays and is relatively imprecise in its measure of the number of envelopes present.
Miller, U.S. Pat. No. 3,968,364 discloses an apparatus wherein the position of a follower arm causes a light circuit to be interrupted when a paper tray becomes sufficiently empty. Miller's sensor cannot determine the presence of a tray of paper and is not easily adapted to mount inside existing paper feeding machines, nor does Miller disclose the novel construction of the present invention.